Upvote:2
In most countries, as long as your new passport isn't issued, and your current passport is still valid, that's the one you can (and have to use). However different countries have different rules about pickup of a new passport. My own requires me to show up in person, and hand over the old one. So unless we know which country you're talking about, it's gonna be difficult to answer.
Besides, talking to the passport issuing authority is probably the best solution. They know.